In the case of conventional vertical MOSFETs, the maximum donor concentration [ND] in an n−-region and hence also the electrical conductivity of the n−-region is governed by the required blocking capability, and vice versa. In the event of an avalanche breakdown, the approximately 1.5×1012 cm−2 donors are ionized, and find their opposite charge in the acceptor charge of the p-conductive region of the MOSFET structure. If the aim is to allow a higher donor concentration, then opposite charges for the donor atoms in the n−-region must be found, for example in the same conductor plane. In the case of MOS field plate transistors with a trench structure, as are known from the document U.S. Pat. No. 6,573,558 B2, this is achieved by means of the charge carriers in the field plate. In the case of compensation components, such as “CoolMOS”, which have n−-regions and p-regions arranged alternatively in cells, this is achieved by means of acceptors in the p-regions as opposite charges.
In this context, the expression an n−-region or p−-region is understood as meaning an area of a semiconductor component which is lightly doped and has an impurity concentration [ND] or [NA] below[ND] or [NA]≦5×1015 cm−3, respectivelywhere [ND] is the donor concentration and [NA] is the acceptor concentration. In compensation components and components according to the present invention, this area can also be extended up to 1×1017 cm−3. The expression an n-region or p-region means an area of a semiconductor component with medium doping and having an impurity concentration between5×1015 cm−3≦[ND] and [NA]≦1×1018 cm−3, respectively.
An n+-region or p+-region means an area of a semiconductor component which is heavily doped and has an impurity concentration above1×1018 cm−3≦[ND] and [NA], respectively.
If the aim is to improve the electrical conductivity of an n−-region in the case of compensation components, such as “CoolMOS”, further, then the compensation level must be set ever more accurately. This is now reaching the limits of technical feasibility. The MOS field plate transistors which are known from U.S. Pat. No. 6,573,558 B2 with a trench structure in contrast have the disadvantage that the entire reverse voltage is dropped at the drain-side end to the n−-region, so that very thick isolation layers are required. A continuous load of 600 V would require SiO2 with a thickness of about 4-6 μm, thus leading to a relatively large structure grid and to considerable technological problems.
Semiconductor devices with a trench structure are also known from the documents U.S. Pat. No. 4,893,160 and U.S. Pat. No. 5,282,018. In these trench structures, avalanche breakdowns in the lightly doped epitaxial area between a gate arrangement in the trench structure and a drain area with a heavily doped substrate are avoided by means of medium to heavily doped zones in the area of the trench bases. Further semiconductor devices with a trench structure are known from the document U.S. Pat. No. 6,608,350 B2. Known trench structures such as these can be used to produce a high-voltage transistor with a low forward resistance on an n+-conductive semiconductor substrate with a lightly doped semiconductor body area on the n+-conductive semiconductor substrate, by defusing compensation regions out of the trench structure into the lightly doped semiconductor body area. The trench can be filled with a dielectric or with a highly resistive material, as is also described in DE 19848828 C2.
The above forward resistance Ron·A and the breakdown voltage of a high-voltage-resistant semiconductor component for a power transistor are linked by the doping and length and the thickness of a drift path, that is to say of the lightly doped n−-region which mainly provides the blocking voltage. High doping and a short drift path mean a low forward resistance, but also a low breakdown voltage. Conversely, light doping and a long drift path are required for a high breakdown voltage, which results in a high forward resistance Ron·A.
The German Patent Application DE 10 2004 007 197.7 describes a semiconductor device in which significantly higher drift path doping is made possible by means of layers which are arranged parallel to the drift path and are composed of a material with a high dielectric constant, which is referred to in the following text as a high-k material (high dielectric constant material), thus resulting in a considerably lower forward resistance. With typical trench widths and widths of the n−-region in the region of a few micrometers, forward resistance values Ron·A which are nowadays better than in the case of “CoolMOS” by a factor of at least 3 can be achieved for 600 V components. A transition from a material with a high dielectric constant to a material with a low dielectric constant such as silicon is located on the lower face of the high-k material layers. This is associated with a corresponding sudden change in the normal component of the electrical field strength E, because this field component is described by:εhk=Ehk=εSiESi,where εhk is the high dielectric constant of the trench material or of the high-k material, Ehk is the field strength at the boundary surface in the material with the high dielectric constant, εSi is the dielectric constant of the silicon and ESi is the field strength in the adjacent silicon. Since the field strength Ehk in the high-k region typically in its own right amounts to half the breakdown field strength of the semiconductor material, the field strength ESi in the semiconductor located underneath this also rises, with a relative dielectric constant of the high-k region of even only 50 to well above the breakdown field strength of the silicon as the semiconductor material, so that the desired blocking capability cannot be achieved in the proposed structures unless the region which is filled with a high-k material, or the filled trench, achieves the transition to the heavily doped n+-region of the heavily doped substrate very precisely, which is technologically scarcely feasible, but has been found to be disadvantageous in the previous technology.
Another critical case of such high-voltage-resistant semiconductor component structures occurs when the high-k region extends too far into the heavily doped n+-semiconductor region of the substrate. This results in a field strength peak at the transition from the n−-doped drift path to the heavily doped region, and this likewise reduces the blocking capability. These high-voltage-resistant semiconductor components are therefore subject to the problem that the high-k region must end as precisely as possible at a heavily doped region of the semiconductor substrate, which, in technological terms, is an object which can be achieved only with difficulty, not least because the trench structures for the high-k regions are incorporated using technologies such as laser ablation or plasma etching, which are not suitable for the removal of material being stopped between lightly doped epitaxial layer areas and heavily doped substrate areas.